Thank you for the answers that fully explains what I needed so far.
What’s wrong with it, unless you had an agreement that you own all code after job completion then you have no chance at fighting this.
It is quite normal for clients to engage developers to do custom work and then release it on codecanyon or other marketplaces. Chances are that what ever you paid for the customization doesn’t barely cover the development costs. Not knowing what you paid, paying 4 times 30 bucks is 120. That barely covers it for sure.
There is no article to share, just use common sense 